6566 - Add 'run_until_cancelled' to 'tokio_util::sync::CancellationToken'

[tglane]

Motivation

Citing from #6566:

Give how (seemingly) often a tokio_util::sync::CancellationToken gets used as follows:
tokio::select! {
_ = token.cancelled() => { /* something / }
other => other_future => { / something else */ }
}
I propose adding a method to CancellationToken that takes a Future, runs it to completion, and returns its output — unless the token is cancelled before the future completes, in which case the future is dropped. I feel the best type for the result of this composed future would be an OptionFut::Output, with None indicating cancellation.

Solution

I added the described function to tokio_util::sync::CancellationToken and added a test case to the loom tests.
I also had to add the macros feature of Tokio to get access to the select! macro. I thought about using select! provided by futures_util but that would require WaitForCancellationFuture to implement FusedFuture so I had to stick with the tokio one. But maybe this is a not worthy trade off just for a convenience function?

This would close #6566

[Darksonn]

Looks pretty good overall!

[Darksonn]

Looks good to me. I only have two remaining things, and otherwise this is ready to be merged.

[Darksonn]

Thank you.